Background writer process

This first part of the background writer does no syncing at all.
It's only purpose is to keep the LRU heads clean so that regular
backends seldom to never have to call write().

Jan

src/backend/postmaster/postmaster.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/postmaster/postmaster.c,v 1.348 2003/11/11 01:09:42 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/postmaster/postmaster.c,v 1.349 2003/11/19 15:55:07 wieck Exp $
  *
  * NOTES
  *
@@ -104,6 +104,7 @@
 #include "storage/pg_shmem.h"
 #include "storage/pmsignal.h"
 #include "storage/proc.h"
+#include "storage/bufmgr.h"
 #include "access/xlog.h"
 #include "tcop/tcopprot.h"
 #include "utils/guc.h"
@@ -224,7 +225,8 @@ char	   *preload_libraries_string = NULL;
 /* Startup/shutdown state */
 static pid_t StartupPID = 0,
 			ShutdownPID = 0,
-			CheckPointPID = 0;
+			CheckPointPID = 0,
+			BgWriterPID = 0;
 static time_t checkpointed = 0;
 
 #define			NoShutdown		0
@@ -298,6 +300,7 @@ __attribute__((format(printf, 1, 2)));
 
 #define StartupDataBase()		SSDataBase(BS_XLOG_STARTUP)
 #define CheckPointDataBase()	SSDataBase(BS_XLOG_CHECKPOINT)
+#define StartBackgroundWriter()	SSDataBase(BS_XLOG_BGWRITER)
 #define ShutdownDataBase()		SSDataBase(BS_XLOG_SHUTDOWN)
 
 
@@ -1055,6 +1058,17 @@ ServerLoop(void)
 			}
 		}
 
+		/*
+		 * If no background writer process is running and we should
+		 * do background writing, start one. It doesn't matter if
+		 * this fails, we'll just try again later.
+		 */
+		if (BgWriterPID == 0 && BgWriterPercent > 0 &&
+				Shutdown == NoShutdown && !FatalError && random_seed != 0)
+		{
+			BgWriterPID = StartBackgroundWriter();
+		}
+
 		/*
 		 * Wait for something to happen.
 		 */
@@ -1478,6 +1492,13 @@ processCancelRequest(Port *port, void *pkt)
 								 backendPID)));
 		return;
 	}
+	else if (backendPID == BgWriterPID)
+	{
+		ereport(DEBUG2,
+				(errmsg_internal("ignoring cancel request for bgwriter process %d",
+								 backendPID)));
+		return;
+	}
 	else if (ExecBackend)
 		AttachSharedMemoryAndSemaphores();
 
@@ -1660,6 +1681,13 @@ SIGHUP_handler(SIGNAL_ARGS)
 		SignalChildren(SIGHUP);
 		load_hba();
 		load_ident();
+
+		/*
+		 * Tell the background writer to terminate so that we
+		 * will start a new one with a possibly changed config
+		 */
+		if (BgWriterPID != 0)
+			kill(BgWriterPID, SIGTERM);
 	}
 
 	PG_SETMASK(&UnBlockSig);
@@ -1692,6 +1720,8 @@ pmdie(SIGNAL_ARGS)
 			 *
 			 * Wait for children to end their work and ShutdownDataBase.
 			 */
+			if (BgWriterPID != 0)
+				kill(BgWriterPID, SIGTERM);
 			if (Shutdown >= SmartShutdown)
 				break;
 			Shutdown = SmartShutdown;
@@ -1724,6 +1754,8 @@ pmdie(SIGNAL_ARGS)
 			 * abort all children with SIGTERM (rollback active transactions
 			 * and exit) and ShutdownDataBase when they are gone.
 			 */
+			if (BgWriterPID != 0)
+				kill(BgWriterPID, SIGTERM);
 			if (Shutdown >= FastShutdown)
 				break;
 			ereport(LOG,
@@ -1770,6 +1802,8 @@ pmdie(SIGNAL_ARGS)
 			 * abort all children with SIGQUIT and exit without attempt to
 			 * properly shutdown data base system.
 			 */
+			if (BgWriterPID != 0)
+				kill(BgWriterPID, SIGQUIT);
 			ereport(LOG,
 					(errmsg("received immediate shutdown request")));
 			if (ShutdownPID > 0)
@@ -1877,6 +1911,12 @@ reaper(SIGNAL_ARGS)
 			CheckPointPID = 0;
 			checkpointed = time(NULL);
 
+			if (BgWriterPID == 0 && BgWriterPercent > 0 &&
+				Shutdown == NoShutdown && !FatalError && random_seed != 0)
+			{
+				BgWriterPID = StartBackgroundWriter();
+			}
+
 			/*
 			 * Go to shutdown mode if a shutdown request was pending.
 			 */
@@ -1983,6 +2023,8 @@ CleanupProc(int pid,
 				GetSavedRedoRecPtr();
 			}
 		}
+		else if (pid == BgWriterPID)
+			BgWriterPID = 0;
 		else
 			pgstat_beterm(pid);
 
@@ -1996,6 +2038,7 @@ CleanupProc(int pid,
 	{
 		LogChildExit(LOG,
 				 (pid == CheckPointPID) ? gettext("checkpoint process") :
+				 (pid == BgWriterPID) ? gettext("bgwriter process") :
 					 gettext("server process"),
 					 pid, exitstatus);
 		ereport(LOG,
@@ -2044,6 +2087,10 @@ CleanupProc(int pid,
 		CheckPointPID = 0;
 		checkpointed = 0;
 	}
+	else if (pid == BgWriterPID)
+	{
+		BgWriterPID = 0;
+	}
 	else
 	{
 		/*
@@ -2754,6 +2801,8 @@ CountChildren(void)
 	}
 	if (CheckPointPID != 0)
 		cnt--;
+	if (BgWriterPID != 0)
+		cnt--;
 	return cnt;
 }
 
@@ -2827,6 +2876,9 @@ SSDataBase(int xlop)
 			case BS_XLOG_CHECKPOINT:
 				statmsg = "checkpoint subprocess";
 				break;
+			case BS_XLOG_BGWRITER:
+				statmsg = "bgwriter subprocess";
+				break;
 			case BS_XLOG_SHUTDOWN:
 				statmsg = "shutdown subprocess";
 				break;
@@ -2883,6 +2935,10 @@ SSDataBase(int xlop)
 				ereport(LOG,
 					  (errmsg("could not fork checkpoint process: %m")));
 				break;
+			case BS_XLOG_BGWRITER:
+				ereport(LOG,
+					  (errmsg("could not fork bgwriter process: %m")));
+				break;
 			case BS_XLOG_SHUTDOWN:
 				ereport(LOG,
 						(errmsg("could not fork shutdown process: %m")));
@@ -2895,19 +2951,22 @@ SSDataBase(int xlop)
 
 		/*
 		 * fork failure is fatal during startup/shutdown, but there's no
-		 * need to choke if a routine checkpoint fails.
+		 * need to choke if a routine checkpoint or starting a background
+		 * writer fails.
 		 */
 		if (xlop == BS_XLOG_CHECKPOINT)
 			return 0;
+		if (xlop == BS_XLOG_BGWRITER)
+			return 0;
 		ExitPostmaster(1);
 	}
 
 	/*
 	 * The startup and shutdown processes are not considered normal
-	 * backends, but the checkpoint process is.  Checkpoint must be added
-	 * to the list of backends.
+	 * backends, but the checkpoint and bgwriter processes are.
+	 * They must be added to the list of backends.
 	 */
-	if (xlop == BS_XLOG_CHECKPOINT)
+	if (xlop == BS_XLOG_CHECKPOINT || xlop == BS_XLOG_BGWRITER)
 	{
 		if (!(bn = (Backend *) malloc(sizeof(Backend))))
 		{